Color display system



July 28, 1970 R. E. SMITH 3,522,368

COLOR DISPLAY SYSTEM Filed April 10, 1967 FIG. 1

fl/GH VOLTAGE M 4MP; lF/EE 7 Na 5a wea-E 58 ///c// Mu r46: 57

OUECE 75 pl/OSPHOE Seed-EN United States Patent 3,522,368 COLOR DISPLAYSYSTEM Robert E. Smith, Richardson, Tex., assignor to Texas InstrumentsIncorporated, Dallas, Tex., a corporation of Delaware Filed Apr. 10,1967, Ser. No. 629,433 Int. Cl. H04n 9/12, 9/22 US. Cl. 1785.4 8 ClaimsABSTRACT OF THE DISCLOSURE This invention relates to color displaysystems and more particularly to apparatus for applying differentelectron accelerating voltages to a phosphor viewing screen.

Various prior art color display systems have been proposed in which thecolor of light produced by the phosphor screen of a kinescope is variedby varying the energy or velocity of the electrons impinging upon thescreen. One proposed method of rendering prosphors differentlyresponsive to electrons of different energies is to coat or overlaycertain of the different color phosphors with a barrier layer so thatonly electrons having energies of at least a predetermined level willexcite the phosphor. To obtain a multicolor display by this method, itis then necessary to apply different accelerating voltages to thephosphor screen as the screen is scanned by an electron beam. Thedifferent voltages must be applied in proper sequence and at arelatively rapid rate, e.g., at a line-sequential rate.

Among the several objects of the present invention may be noted theprovision of apparatus for applying different accelerating voltages to aphosphor screen in a color display system; the provision of suchapparatus which can change voltage levels at a relatively rapid rate,e.g., at a line-sequential rate; the provision of such apparatus whichis highly reliable; and the provision of such apparatus which isrelatively simple and inexpensive. Other objects and features will be inpart apparent and in part pointed out hereinafter.

Briefly, apparatus according to the present invention is useful inapplying different electron accelerating voltages to a phosphor viewingscreen of the type which emits light of different colors when struck byelectrons of different energies. The apparatus includes a sourceproviding direct current at a relatively high voltage between a positiveand a negative terminal and a vacuum tube having a high voltage blockingcapability. The tube has an anode, a cathode and a grid, conductionbetween the anode and the cathode being variable as a function of thegrid-cathode voltage. The anode of the tube is connected to the positiveterminal of the source through a voltage dropping impedance and isconnected also to the viewing screen. The cathode of the tube isconnected to the negative terminal of the source through thecollector-emitter circuit of a transistor. The apparatus includes alsomeans for varying conduction between the collector and emitter of thetransistor thereby to vary also conduction between the anode and cathodeof the tube. Accordingly, different electron accelerating voltages areapplied to the screen by the 3,522,368 Patented July 28, 1970 Fee anodeof the tube for accelerating electrons to at least two differentenergies for producing light of respective colors.

In the accompanying drawings in which one of various possibleembodiments of the invention is illustrated,

FIG. 1 is a diagrammatic illustration of a color display systemincluding apparatus according to this invention for applying differentelectron accelerating voltages to the phosphor viewing screen of akinescope; and

FIG. 2 is a schematic circuit diagram illustrating in greater detail theapparatus for providing different electron accelerating voltages.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

f Referring now to FIG. 1, there is indicated at 11 a color kinescope ofa type with which the present invention is useful. Kinescope 11 includesa conventional glass envelope 13 having a screen portion 15 and a neckportion 17. Coated on the inner surface of the screen portion 15 is aphosphor screen or layer 19 which includes phosphors which emit light ofdifferent colors when struck by electrons of different energies.Phosphor screen 19 may, for example, be constituted by a mixture ofthree different kinds of phosphor particles a first of which emits redlight when energized by elec trons having energies above a first,relatively low predetermined level; a second of which emits cyan lightwhen energized by electrons having energies above a second orintermediate level; and a third of which emits blue light when energizedby electrons having energies above a third, relatively highpredetermined level. As

,'screen 19 is subjected to impinging electrons of increasing energies,the three kinds of phosphors are cumulatively energized so that thescreen emits red light when struck by electrons at the relatively lowlevel; warm, substantially achromatic light when struck by electrons atthe intermediate energy level; and cool, substantially achromatic lightwhen struck by electrons at the relatively high energy level. Suchunsaturated color image displays are described in greater detail inapplication 450,705, 'filed Apr. 26, 1965, now abandoned in favor ofapplication 6l4,362, now Pat. No. 3,337,229. Images presented by suchdisplays appear to have a relatively wide range of hues subjectivelyhaving a greater saturation than that which is actually present in thecolorimetric sense. Methods of preparing phosphors useful in making sucha screen are disclosed in application Ser. No. 459,- 582, filed May 28,1965, now Pat. No. 3,408,223.

Over phosphor screen 19 is deposited a film 21 of aluminum which isconductive and yet is also thin enough to be substantially electronpermeable. By means of film 21, suitable electron accelerating voltagesmay be applied to the phosphor screen 19.

Within the neck portion 17 of envelope 13 there is mounted aconventional electron gun 23 for emitting a beam of electrons directedtoward phosphor screen 19. Gun 23 includes a cathode 25 for providingthe electrons which are formed by the gun into a beam and a grid 27 formodulating the beam current or number of electrons emitted by the gun.For the purpose of the example described herein, it is assumed that thiscolor display system is operated in a line-sequential mode. For thispurpose, a three-color, line-sequential video signal is applied to grid27 by means described hereinafter for varying the electron beam current.The video signal thus controls the instantaneous brightness of the lightproduced by the beam on phosphor screen 19. It should be understood,however, that other modes of presentation, such as field-sequential, mayalso be employed by appropriately varying the different voltageswitching rates described hereinafter and by applying a correspondinglyswitched video signal to grid 27.

Electrons emitted from gun 23 are accelerated by the voltage applied toscreen 19 and pass through the magnetic influence of a deflection yoke29. Yoke 29 is energized in conventional manner to deflect the beam ofelectrons over the screen 19 in a scanning raster comprising a series ofgenerally parallel horizontal lines.

Video signals representing color records of the red, green and bluecomponents of the composite full color image to be displayed areapplied, through respective leads 35, 37 and 39, to gate circuitsindicated at 41 which are operative to pass selected ones of thesignals. Gate circuits 41 are operated under the control of a syncsignal, provided through a lead indicated at 45, to pass selected onesof the video signals in sequence to an output lead 47. The sync signalprovided at lead 45 may, for example, be constituted by the conventionalNTSC horizontal synchronization pulse so that the composite video signalprovided at lead 47 changes from one color to the next on successivelines, thereby providing a line-sequential video signal in conventionalmanner.

The gate circuits 41 also operate to supply to an output lead 51 aseries of DC. voltage levels in predetermined sequence constituting astaircase waveform as indicated graphically adjacent to the lead 51. Thesequence in which the different voltage levels are applied to lead 51 issynchronized with the application of the different video signals to lead47 so that the same voltage level is always present at the lead 51 whena given one of the video signals is being applied to the lead 47.

The staircase voltage waveform provided to lead 51 is amplified andsuperimposed upon a high voltage DC. bias by the amplifier and biassource circuits indicated generally at 55. The resultant high voltagestepped waveform is then applied to screen 19 through a lead 57 therebyto provide a sequence of different accelerating voltages for electronsemitted by the gun 23. Electrons emitted by gun 23 during the differenttime intervals corresponding to the three different voltage levels ofthe staircase waveform are thus accelerated to different energy levelsbefore reaching phosphor screen 19. The respective voltages and energylevels are chosen in relation to the thresholds or level-sensitivecharacteristics of the phosphors which make up screen 19 so that thelower energy electrons excite only the red phosphor; the intermediateenergy electrons excite both the red and cyan phosphors thereby causingwarm, substantially achromatic light to be emitted; and the high energyelectrons cause all of the phosphors, including the blue, to beenergized thereby causing cool, substantially achromatic light to beemitted.

As the stepped voltage waveform applied to screen 19 is synchronized inrelation to the sequence in which video signals representing differentcolors are applied to gun 23, the beam current is thus modulated toreproduce the various image components represented by the differentvideo signals in respective colors. As noted previously, the changesbetween successive colors are assumed to take place at a line-sequentialrate.

The circuitry which provides the different electronic acceleratingvoltages to screen 19 is shown in greater detail in FIG. 2. DC. at asuitably high voltage for electron accelerating purposes is provided bya bias source as indicated at 58 having a positive supply terminal 59and a negative supply terminal 60 which is grounded. For modulating thisvoltage to provide different voltage accelerating levels there isprovided a vacuum tube 63 having substantial high voltage blockingcapability. Tube 63 comprises an anode 65, a cathode 67 and a grid 69;As is understood by those skilled in the art, conduction between theanode and cathode of tube 63 varies as a function of the grid-cathodevoltage. The anode 65 is connected to the positive terminal of the highvoltage bias source 58 through a voltage dropping impedance which,

in the example illustrated, comprises a resistor R1. An inductor mayalso be used. Anode is also connected to lead 57 which, as describedpreviously, is connected to the screen 19. The electron acceleratingvoltage provided at screen 19 will thus vary as a function of thevoltage drop produced across resistor R1 by current drawn by conductionthrough the tube 63.

As is understood by those skilled in the art, a vacuum tube such as thatindicated at 63 exhibits a capacitive coupling between the anodes andthe grid thereof so that voltage changes at the anode are coupled to thegrid. This capacitive coupling substantially reduces the switching timewhich may be provided by such a tube particularly if conduction in thetube is controlled from the grid.

To obtain rapid switching between voltage levels in the apparatusillustrated in spite of this capacitive load ing and the capacitive loadof screen 19, conduction through tube 63 is controlled from its cathodeby an NPN transistor as indicated at Q1. The tube grid 69 is connectedto ground through a resistor R2 which maintains the grid nominally atDC. ground potential and a capacitor C1 which shunts all AC. orswitching signals to maintain the grid at A.C. ground also.

The tube cathode 67 is connected to ground, and thus also to thenegative terminal of source 58, through the collector-emitter circuit oftransistor Q1, the collector of the transistor being connected to thecathode and the emitter to ground. The relatively low level staircasevoltage provided at lead 51 by the gate circuits 41 is coupled, througha capacitor C2, to the base electrode of the transistor Q1 forcontrolling conduction in its collector emitter circuit. The baseterminal is normally biased to ground potential by a resistor R3connecting the base to ground.

In operation, the low voltage signal coupled to the base terminal causesconduction through the collector-emitter circuit of transistor Q1 tovary in a stepwise fashion and conduction through the transistor iscoupled to the cathode of tube 63 causing conduction between the anodeand the cathode of the tube to vary similarly despite the interelectrodecapacitance of the tube and the capacitance of the load. The electronaccelerating voltage applied to the screen 19 is thus rapidly switchedbetween the different levels thereby to produce light of differentcolors at the screen in proper synchronization with the video signalsapplied to gun 23 so that the color record signals are displayed inrespective distinct colors over the entire width of the screen 19.

In view of the above it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

What is claimed is:

1. In a color display system having a phosphor viewing screen whichemits light of difierent colors when struck by electrons of differentenergies; apparatus for applying different electron acceleratingvoltages to said screen comprising:

a source having a positive terminal and a negative terminal forproviding direct current at a relatively high voltage;

a vacuum tube having a substantial high voltage blocking capability,said tube having an anode, a cathode and a grid, conduction between saidanode and cath ode being variable as a function of the grid-cathodevoltage;

a voltage dropping impedance connecting said anode to said positiveterminal;

means for connecting said anode to said screen;

means connecting said grid to said positive terminal,

a transistor having a collector, an emitter and a base for controllingconduction between said collector and said emitter;

means for connecting said cathode to said negative terminal through thecollector-emitter circuit of said transistor; and

means for varying the voltage at said base between a plurality of levelsto vary the conduction between said collector and said emitter andthereby also vary conduction between said anode and said cathode wherebydifferent electron accelerating voltages are applied to said screen foraccelerating electrons to at least two different energies.

2. Apparatus as set forth in claim 1 wherein said means for connectingsaid grid to said positive terminal includes a resistor for establishingthe DC bias on said grid and a capacitor connected across said resistorfor shunting A.C. signals away from said grid.

3. Apparatus as set forth in claim 1 wherein said voltage droppingimpedance is a resistor.

4. Apparatus as set forth in claim 1 wherein said transistor is of theNPN conductivity type and has its collector connected to the cathode ofsaid tube.

5. Apparatus as set forth in claim 1 including means for applying asignal having a stepped waveform to the base of said transistor therebyto vary conduction between its collector and emitter.

6. A line-sequential color display system comprising:

a phosphor viewing screen which emits light of differ-- ent colors whenstruck by electrons of difierent energies;

electron gun means for generating a beam of electrons directed at saidscreen;

deflection means for scanning said beam over said screen in a successionof generally parallel line sweeps;

means for modulating the beam current on successive line sweeps inaccordance with respective color records;

a source having a positive terminal and a negative terminal forproviding direct current at a relatively high voltage;

a vacuum tube having a substantial high voltage blocking capability,said tube having an anode, a cathode and a grid, conduction between saidanode and cathode being variable as a function of the grid-cathodevoltage;

a voltage dropping impedance connecting said anode to said positiveterminal;

means for connecting said anode to said screen;

an NPN transistor having a collector, an emitter and a base forcontrolling conduction between said collector and said emitter;

means connecting said grid to said positive terminal;

means for connecting said collector to said cathode and for connectingsaid emitter to said negative terminal; and

means for varying conduction between said collector and said emitter onsuccessive line sweeps in synchronism with the modulation of said beamcurrent thereby to vary also conduction between the anode and cathode ofsaid tube whereby a stepped voltage waveform is applied at high voltageto said screen for accelerating electrons to different energiescorresponding to said different colors on successive line sweeps therebyto display each of said records in light of a respective color.

7. A line-sequential color display system as set forth in claim 6wherein said screen includes a first phosphor which emits substantiallyred light when energized by electrons having energies above a first,relatively low predetermined level, a second phosphor which emitssubstantially cyan light when energized by electrons having energiesabove a second level which is higher than said first level, and a thirdphosphor which emits substantially blue light when energized byelectrons having energies above a third level which is higher than saidsecond level.

8. A line-sequential color display system as set forth in claim 7including means for applying a signal having a three-level steppedwaveform to the base of said transistor thereby to vary conduction inthe collector-emitter circuit of said transistor and the anode-cathodecircuit of said tube whereby three dilferent accelerating voltages areapplied in sequence to said screen.

References Cited UNITED STATES PATENTS 3,312,781 4/1967 Land. 3,330,9907/ 1967 Guillette. 3,396,233 8/1968 Kagan. 3,413,410 11/1968 Farmer.

RICHARD MURRARY, Primary Examiner

